Reader: mark throw blocks as cold

[AndyAyersMS]

See TODO.

We know dynamically these blocks are rarely hit, so they should be moved out of line by block layout or put into a cold code section if we support code splitting.

[JosephTremoulet]

Probably (hopefully?) this is a simple application of MDBuilder::createBranchWeights.

[mjsabby]

If the exception were to be implemented as a function, one could set addFnAttr(llvm::Attribute::Cold); which I think gives a low weight to the edges into blocks post-dominated by said function. Seems like createBranchWeights will do the job.